Airports are usually equipped with secondary radars for interrogating the aircraft moving around in their vicinity. The aircraft process the interrogation messages sent from the ground using onboard transponders. Conventionally, the secondary radars are equipped with directional antennas pointing towards the aircraft to be interrogated, so as to focus the power of the signals carrying the interrogation messages and thus facilitate their onboard processing.
A problem in decoding the interrogation messages arises when the power of the signals sent by the secondary radars is to be reduced or when trying to decode these signals at greater distance than the initially planned distance. For example, it may be desirable to widen the area covered by a radar on the ground without increasing the power of the signals sent; there may also be a desire to send the interrogation messages from the ground with an omnidirectional antenna to an aircraft. The power received by the transponder is then insufficient to be able to decode the messages with the current equipment.
To compensate for the link budget losses between the interrogator and the transponder, it is possible to increase the power of the interrogator. This solution is, however, rarely applicable for reasons of operational constraints and/or installation costs, because it entails significant hardware modifications.
Among the existing interrogation message formats, the mode S is more particularly concerned, because it is a selective interrogation mode widely used in civil aviation. A message in mode S format, represented in FIG. 1, comprises a preamble 101 followed by a phase inversion 103 and data 105 transmitted by differential phase modulation, which modulation will hereinafter be designated by the acronym DPSK, standing for “Differential Phase Shift Keying”. The preamble 101 comprises two pulses 111, 112 separated in time by a known duration.
The American patent published under the reference U.S. Pat. No. 5,089,822 presents a method for detecting the preamble of an interrogation message in mode S performing a comparison between an expected pattern of two pulses with a threshold of the received signal. However, the use of this method results in numerous false detections when the signal-to-noise level of the received signal is low. This same patent U.S. Pat. No. 5,089,822 proposes detecting the phase inversion 103. However, this simple phase inversion detection may prove imprecise for a time synchronization used to perform a correct decoding of the data 105.